One of the usual problems when starting an electric machine is that the inrush current is much higher than the rated current, which can be harmful to the machine and to the mains or electrical power lines.
In the specific case of connection of transformers, when a transformer is powered on, transient currents can appear with values up to several times the transformer's rated current. The duration and intensity of this current is determined by constructive aspects of the transformer, the line resistance and mainly the residual flux of the transformer and the connection instant.
Depending on the instant the power supply is applied to the transformer, the magnetic flux wave can reach a peak value higher than twice its maximum value in normal operation conditions. This makes flux value exceed by far the “knee” of the magnetization hysteresis loop of the transformer.
The current in the primary winding required to produce that flux value can be up to several times the rated current of the transformer. This is the inrush current or starting current of the transformer.
The problems caused by these high current values on the physical structure of the transformer are well known, and go up to the point to decide not to deenergize the transformer to avoid the stress caused when it is energized again, which causes the situation that the transformer is permanently connected to the mains.
There are several technical solutions concerning methods to limit the inrush current appearing during the connection of a transformer to the mains by means of the premagnetization of the magnetic core of the primary winding of the transformer and the control of the phase angle of the mains voltage.
U.S. Pat. No. 5,216,567 A discloses a transformer inrush current limiting device in which there are certain magnetic conditions in the magnetic core of the primary winding and the phase angle of the mains voltage is controlled. This document provides a device of the above-mentioned type which without any design intervention of the power supply unit allows making the full power available, without noticeable retardation, after the cut-in of the power supply, and to suppress cut-in current peaks.
The solution consists of a capacitor on the one hand provided parallel to a transformer, and on the other hand arranged in series with an alternating current switch, a charging resistor and a rectifier. When a mains-off recognition circuit detects the incidental or intended shutoff of the mains, the alternating switch is fired and after several milliseconds the discharge of the capacitor towards the magnetic core of the transformer primary winding starts.
The discharge of the capacitor provides for certain known magnetization conditions of the magnetic core of the transformer primary winding, so the transformer can be connected to the mains at a preselectable phase angle between 90° and 0° before a zero passage of the mains voltage, i.e., between 90° and 180° in the positive or negative half cycle of the sinusoidal signal. In this way the cut-in current is limited to the charge current value before the zero passage of the mains voltage.
However, the device described in this document is connected to the primary winding by means of power electronics components which are unable of withstanding high voltages, which make them not applicable to high voltage. The cut-in current is limited but not suppressed and the premagnetization of the magnetic core of the transformer primary winding is made through the discharge of a capacitor.
U.S. Pat. No. 5,642,025 A discloses a process and an apparatus for the reduction of the inrush current of a three phase transformer connected to a rotary load in which there are certain magnetic conditions in each line of the magnetic core of the primary winding and the phase angle of the mains voltage is controlled. This document provides a process and an apparatus of the above-mentioned type by means of which a rotary load can be connected even in the case of variable charge conditions with a considerably reduced inrush current, preferably practically negligible.
The provided solution consists of creating a magnetization pattern in the lines of the magnetic core of the primary winding that allows to know the magnetic conditions of each line at each instant, so it can be connected at the right time, thus avoiding an excess inrush current.
The disclosed apparatus comprises servo components, connected to a control which contains a sequence control and a phase angle control, which can be controlled in such a manner that different current flow angles and switching sequences can be realized. The servo components are triacs, thyristors or a combination of electro-mechanical switches and semiconductor switches.
In the case of combination of electro-mechanical switches and semiconductor switches, when the transformer is fully connected to the mains, the semiconductor switches are bridged by mechanical contacts.
This document discloses the use of electro-mechanical switches in combination with semiconductor switches, the semiconductor switches being bridged when the transformer is fully connected to the mains. The semiconductor switches are unable to withstand high voltages so they cannot be applied in high voltage.
A drawback of the prior art systems is that they are not suitable for high voltage, as they are connected to the primary winding by means of a power electronics that does not withstand high voltages.
Accordingly, it would be desirable to obtain a process that limits or suppresses the inrush current during the load connection in high voltage.